In navigation systems for motor vehicles, digital maps are used to input destinations and to guide the driver of the vehicle to a defined destination via a route guidance, for example. The digital maps used for this purpose have roads that are represented by so-called segments. Such segments may be made up of a line element or of a plurality of line elements, which may be straight, for example, whereby the route of a segment approximates the actual route of the road. If the segments meet at an intersection or a junction, these meeting points are represented by so-called nodes.
Because road routings may change or roads may be newly built, it is essential to keep the maps that are in use up-to-date and to update the road topology of the map, which is represented by segments and nodes. An update through wireless methods, such as via bidirectional communication networks, has the disadvantage that the available transmission capacity is only limited and the general availability of such systems also is not always guaranteed. Thus, it is necessary for the wireless updating of the map to be restricted to a currently required region of the map only. In this context, however, a situation may occur in which the updated digital map has an inconsistency at the border between an updated region and a non-updated region. For example, such an inconsistency may be a new road that leads across the border of the regions and is not yet included in the region that has not yet been updated and thus is not continued there. If a navigation system now proposes a suggested route via a segment representing such a street, then problems result at the border between updated and non-updated regions, because the road suddenly ends at the border between the updated region and the non-updated region.
In known concepts, such as in the European project ActMap, it is assumed that the digital maps are updated to the point where a consistent calculation of a route is possible.
The not yet pre-published DE 10 2006 013 297 discloses a method for operating a navigation system in which map data are updated, the updated map data being stored in a memory and additionally map data that have not yet been updated being stored in a safety memory level, so that it is possible to revert to these data, where necessary. The update is performed using bidirectional communication, which is not always available and which is not available everywhere.
However, wireless communication channels for updating digital maps do not have one-hundred-percent availability, and when a communication channel is not available, it is not possible to update the digital map to the necessary extent, and for this reason a calculation of such a route could be impossible or problematic.